1. Field of the Invention
This invention relates to color toners used in the development of electrostatic latent images or in a toner jet system. More particularly, this invention relates to a cyan toner, a magenta toner and a yellow toner which exhibit color reproduction ranges matched to overhead projector (OHP) projection tones having a high chroma and a high transparency and to color tones of process inks even when using heat-and-pressure fixing means in which any oil for preventing high-temperature offset is not used or such an oil is used in a small quantity; and a full-color image-forming method making use of these toners.
2. Related Background Art
In recent years, in proposed full-color copying machines there is commonly used a method in which, using four photosensitive members and one beltlike transfer member, electrostatic latent images formed respectively on the photosensitive members are developed with a cyan toner, a magenta toner, a yellow toner and a black toner to form corresponding toner images and then a transfer medium is so transported as to be held between the photosensitive members and the beltlike transfer member to transfer the toner images in straight pass, followed by fixing to form a full-color image, and a method in which the transfer medium is wound around the surface of a cylindrical transfer member set opposite to a photosensitive member, by the aid of electrostatic force or mechanical action of a gripper or the like, and the steps of development and transfer are carried out four times, followed by fixing to form a full-color image.
As toners used in such full-color copying machines, the toners are required to be well color-mixed in the step of heat-and-pressure fixing, without damaging any color reproducibility and any transparency of overhead projector (OHP) images. Compared with ordinary toners for black-and-white copying machines, toners for full-color images may preferably make use of low-molecular-weight binder resins having sharp-melt properties. However, usually, the use of such binder resins having sharp-melt properties tends to cause a problem on high-temperature anti-offset properties because of a low self-agglomerative force of the binder resins when the toners melt in the step of heat-and-pressure fixing. In ordinary toners for black-and-white copying machines, relatively highly crystalline waxes as typified by polyethylene wax and polypropylene wax are used as release agents in order to improve high-temperature anti-offset properties. For example, these are disclosed in Japanese Patent Publication Nos. 52-3304 and 52-3305 (Japanese Patent Application Laid-Open Nos. 49-065231 and 49-065232) and Japanese Patent Application Laid-Open No. 57-52574. In the toners for full-color images, such release agents may inhibit transparency when images are projected by an OHP, because of their own high crystallizability and a difference in refractive index of the materials of PH sheets, so that the projected images may have low chroma and lightness.
To solve such a problem, toners having a specific storage elastic modulus are proposed.
For example, Japanese Patent Application Laid-Open Nos. 11-84716 and 8-54750 disclose toners having a specific storage elastic modulus at 180° C. or 170° C. However, as for color toners required to have both low-temperature fixing performance and high-temperature anti-offset properties, to have a good fixing performance in the heat-and-pressure fixing means in which any oil for preventing high-temperature offset is not used or such an oil is used in a small quantity, and to have a sufficient color mixing performance, the toners may have too low viscosity and also have not been satisfactory in respect of storage stability in a high-temperature environment.
Japanese Patent Application Laid-Open Nos. 5-249735, 7-92737, 7-234542, 7-295298, 8-234480, 8-278662 and 10-171156 also disclose toners having specific storage elastic moduli. However, in order to attain fixing performance, storage stability and OHP transparency which are ideal for color toners, there has been room for improvement.
To solve the above problem, as disclosed in Japanese Patent Application Laid-Open Nos. 4-149559 and 4-107467, a method is proposed in which a nucleating agent is used in combination with a wax so as to lower the crystallizability of the wax. As also disclosed in Japanese Patent Application Laid-Open Nos. 4-301853 and 5-61238, a method is proposed in which a wax having a low crystallinity is used. As waxes having a relatively good transparency and a low melting point, montan type waxes are available. The use of montan type waxes is disclosed in Japanese Patent Application Laid-Open Nos. 1-185660, 1-185661, 1-185662, 1-185663 and 1-238672. These waxes, however, by no means satisfy all the transparency in OHP and the low-temperature fixing. performance and high-temperature anti-offset properties at the time of heat-and-pressure fixing.
Accordingly, in usual color toners, an oil such as silicone oil or fluorine oil is applied to heat fixing rollers without adding any release agent as far as possible, so as to achieve an improvement in high-temperature anti-offset properties and OHP transparency. However, fixed images thus obtained have excess oil having adhered to their surfaces. This oil may adhere to photosensitive members to cause contamination or the oil may swell fixing rollers to shorten the lifetime of the fixing rollers. In order not to cause any oil streaks on the fixed images, it is necessary to feed oil onto the fixing roller surface evenly and in a constant quantity. This tends to require fixing assemblies having a large size.
Accordingly, in the heat-and-pressure fixing means in which any oil is not used or the oil is used in a small quantity, it is long-awaited to provide a toner having kept offset from occurring and also promising superior transparency of fixed images.
Meanwhile, with an increase in cases in which color copying machines are connected to computers via controllers and used as high-grade color printers, a color management system has come to be proposed which makes color control of the whole system. As a result, specific users have come to strongly demand that the printed images produced by a color printer of an electrophotographic system are identical in tinges with the printed images produced by printing making use of process inks. Thus, there has been a demand for a cyan toner, a magenta toner and a yellow toner which have the same color tones as process inks, and for an image-forming method making use of them.
Some proposals have ever been made on pigments for cyan toners, and known various dyes and pigments exhibiting cyan chromatic color are in wide use, such as C.I. Pigment Blue 15:3, do. 15:4, C.I. Solvent Blue 25, do. 35, do. 68, do. 70 and do. 111.
Meanwhile, in the case of full-color images, colors are reproduced using three chromatic toners consisting of three-primary-color coloring materials, a yellow toner, a magenta toner and a cyan toner, or four color toners consisting of these toners and a black toner added thereto. In order to obtain images having the intended color tones, the balancing with different colors is important, and it is proposed to use the same-color pigments or dyes in combination or to use different-color pigments and/or dyes in combination in order to slightly change the color tone of the cyan toner. For example, Japanese Patent Publication No. 50-777 (Japanese Patent Application No. 47-083365) proposes the use of a cyan pigment and a yellow pigment in combination; Japanese Patent Application Laid-Open No. 61-7844, the use of a cyan pigment and the same-color dye in combination; and Japanese Patent Application Laid-Open No. 62-280779, the use of a cyan pigment and a magenta pigment in combination.
Japanese Patent Application Laid-Open No. 3-276163 also discloses the use of C.I. Pigment Blue 15:3 and C.I. Pigment Green 7 in combination. It, however, does not refer to any ratio of both pigments. Japanese Patent Application Laid-Open No. 2001-5221 still also discloses the use of C.I. Pigment Blue 15:3 and C.I. Pigment Green 36 in combination. The C.I. Pigment Green 36, however, has been replaced with Br, and has had unsatisfactory charge maintenance performance and environmental stability. With regard to fixing performance, too, it has been found necessary to make further improvement. It, however, has been found that a charge control agent is limited to a metal salt of a benzilic acid derivative, and has a drawback in charging stability and fixing performance, as compared with an aromatic carboxylic acid derivative selected from an aromatic oxycarboxylic acid and an aromatic alkoxycarboxylic acid, and a metal compound of the aromatic carboxylic acid derivative, which are described in the present invention.
Some proposals have also ever been made on pigments for magenta toners. In view of superior sharpness and transparency of color and also superior light-fastness, quinacridone pigments have been in wide use.
For example, Japanese Patent Application Laid-Open Nos. 49-27228, 57-54954 and 1-142559 disclose a toner making use of 2,9-dimethdylquinacridone alone. This toner certainly has a superior light-fastness, but cannot be said to be a sufficiently vivid magenta toner. Japanese Patent Application Laid-Open No. 64-9466 discloses that a quinacridone pigment and a xanthene dye or a pigment obtained by making a xanthene dye into a lake are used in combination so as to improve the vividness of toners. This toner has not attained a sufficient vividness, and has had such a problem that it changes in color and images formed may change in color when left standing over a long time.
Japanese Patent Application Laid-Open No. 1-154161 discloses the use of a quinacridone pigment of 0.5 μm or smaller average particle diameter in an attempt to improve the transparency of magenta toners. The transparency of toners depends on pigments, resins and how and to what extent the pigments are dispersed in resins, and any magenta toners having a high transparency have not necessarily been obtained.
Meanwhile, in the case of full-color images, colors are reproduced using three chromatic toners consisting of three-primary-color coloring materials, a yellow toner, a magenta toner and a cyan toner, or four color toners consisting of these toners and a black toner added thereto. In order to obtain images having the intended color tones, the balancing with different colors is important, and it is also attempted to slightly change the color tone of the magenta toner.
For example, Japanese Patent Publication No. 63-18628 (Japanese Patent Application Laid-Open No. 55-048250 discloses a mixture of compounds which contains two types of substituted quinacridones. Japanese Patent Application Laid-Open No. 62-291669 discloses the use of a mixed crystal of 2,9-dimethylquinacridone and unsubstituted quinacridone as a magenta colorant, which is proposed as a colorant having the intended hue and also aiming at an improvement in triboelectric charging performance of toners.
Its color tone has more shifted toward a tinge of yellow as a whole than the case of the use of only 2,9-dimethylquinacridone. However, it is blue-tinged as compared with the hue of magenta inks for offset printing. Thus, there have remained many points to be improved.
Nowadays, as colorants for yellow toners, a large number of colorants are known in the present technical field. For example, as dyes, Japanese Patent Application Laid-Open No. 2-207273 discloses C.I. Solvent Yellow 112; Japanese Patent Application Laid-Open No. 2-207274, C.I. Solvent Yellow 160; and Japanese Patent Application Laid-Open No. 8-36275, C.I. Solvent Yellow 162. As pigments, Japanese Patent Application Laid-Open No. 50-62442 discloses a benzidine type yellow pigment; Japanese Patent Application Laid-Open No. 2-87160, a monoazo type yellow pigment; and Japanese Patent Application Laid-Open No. 2-208662, C.I. Pigment Yellow 120, 151, 154 and 156.
However, colorants for yellow toners conventionally known have had various problems. For example, although dye type colorants commonly have superior transparency, they are inferior in light-fastness, and have a problem in storage stability of images.
Meanwhile, although the above group of pigments have light-fastness superior to that of dyes, they have still a problem in light-fastness, compared with, e.g., quinacridone pigments used for magenta toners or copper phthalocyanine pigments used for cyan toners. There has arisen such a problem that they discolor or conspicuously change in hue in a long-time light exposure test.
In addition, although yellow pigments having superior light-fastness and heat resistance are also available besides the foregoing, they have so strong hiding power as to result in an extremely low transparency, and are unsuitable for full-color image formation.
Japanese Patent Application Laid-Open No. 2-37949 refers to a disazo compound having superior light-fastness and its production process. This is a group of compounds typified by C.I. Pigment Yellow 180, which is one of azo pigments not only having superior light-fastness and heat resistance but also meeting ecological demands.
Yellow toners making use of C.I. Pigment Yellow 180 alone is disclosed in Japanese Patent Application Laid-Open Nos. 6-230607, 6-266163 and 8-262799. Toners having these pigments, however, have a poor coloring power, and in addition can by no means be said to have good transparency. Thus, as their use for full-color image formation, it has been a matter of great urgency for them to be more improved.
Meanwhile, Japanese Patent Application Laid-Open No. 8-209017 discloses an electrophotographic toner in which, in order to solve the above problem, a pigment is made fine-particle to improve the specific surface area of the pigment to improve its transparency and coloring power. However, where the pigment classified as C.I. Pigment Yellow 180 is made fine-particle, it may insufficiently be dispersed in the binder resin included in the toner because of its unavoidably strong self-agglomerative properties. According to studies made by us, toners having pigments with poor dispersibility can hardly achieve charge stabilization and have caused problems of fog and toner scattering.
Japanese Patent No. 2632423 discloses toners produced by kneading and dispersing a group of condensation azo type yellow pigments in resins.
The above toner has achieved the sharpness and clearness of hues and also the improvement in transparency by kneading and dispersing slightly dispersible compounds in an average particle diameter of 0.2 μm or less. However, when viewed as yellow toners for forming highly minute full-color images, the level of pigment dispersibility does not still reach any aimed level. Further, in studies made by us, it is difficult to stabilize charge. Problems such as density decrease and fog have also arisen during extensive operation (running).
Meanwhile, in the case of full-color images, colors are reproduced using three chromatic toners consisting of three-primary-color coloring materials, a yellow toner, a magenta toner and a cyan toner, or four color toners consisting of these toners and a black toner added thereto. In order to obtain images having faithfully reproduced the color tones of process inks in the electrophotographic system and toner jet system, the balance with different colors is very important. Among color toners distributed at present in the market, the yellow toner has a tone most apart from the process inks in actuality. Accordingly, use in combination with a pigment or dye more yellow-tinged than conventional yellow pigments should have been proposed. However, no invention having such an object has ever been found. Also, in that case, taking into account the balancing with cyan color, the reproduction of green color becomes weak. Hence, in a sense of its compensation, use in combination with a pigment or dye more green-tinged than conventional cyan pigments should have been proposed. However, no invention having such an object has ever been found. Also, in that case, making the color tone of magenta constant certainly brings about a great improvement in the reproducibility of red color, but conversely results in a poor reproducibility of blue color. Hence, it is necessary to delicately adjust the color tone of magenta pigments, and use in combination with a pigment or dye achievable of such an object should have been proposed. However, no invention having such an object has ever been found.